The use of turbochargers in internal combustion engines is well known. Turbochargers increase the mass of air supplied to the engine thereby enabling the increase of the power output of the engine. In addition, the efficiency of the engine is increased by the turbocharger's utilization of the thermal energy contained in the engine's exhaust gases.
However, the connection between a turbocharger and the engine has posed various design challenges. For the engine to operate at optimum efficiency, the engine must transfer as much energy as possible from the exhaust gases of the engine to a turbine of the turbocharger, thereby maximizing the boost provided by the turbocharger. Energy is lost from the flow of exhaust gases in the exhaust manifold due to wall friction, area changes in the manifold, and directional changes in the manifold due to flow separation and the creation of secondary flows. All three of these causes of energy loss are typically present in the area of the exhaust manifold where it joins the turbocharger, i.e. the exhaust manifold flange. Therefore, an optimal exhaust manifold and exhaust manifold flange design is successful in minimizing these energy losses.
When energy is lost from the exhaust gas flow through the exhaust manifold flange and in the area of the exhaust manifold near the flange, the energy is typically transformed via convection into thermal energy in the exhaust manifold and flange. Therefore, if the design of the exhaust system reduces the amount of heat absorption from the exhaust gas flow by the exhaust manifold and the exhaust manifold flange, the energy transferred to the turbine of the turbocharger is increased and the efficiency of the engine is improved. In addition, the exhaust manifold and exhaust manifold flange design that reduces the heat absorption of the manifold and flange increases the operating life of the manifold flange and turbocharger. When the exhaust manifold flange absorbs an excess amount of thermal energy, the flange typically develops stress cracks. Such cracking results in failures and not only requires replacement of the flange and/or a portion of the exhaust manifold with which the flange is integral, but it can also cause damage to the turbine of the turbocharger. For example, debris from the cracked and failed manifold passes into the turbine of the turbocharger. This problem of cracking exhaust manifold flanges has been exacerbated by the recent dramatic increases in internal combustion engine exhaust gas temperatures caused by the industry's drive to increase the power output of engines while reducing unwanted emissions.
An exhaust manifold flange must also have the structural integrity to support a rigid connection with a turbocharger. This rigid connection reduces vibrations between the turbocharger and the flange and ensures that a good seal is maintained between the turbocharger and flange. In addition, the connection between the exhaust manifold flange and the turbocharger is typically the only rigid connection between the turbocharger and the engine. All other connections between the turbocharger and the engine are flexible so that no significant forces will be applied to the turbocharger from thermal expansion of the turbocharger, the engine or the connections. Therefore, an exhaust manifold flange must be capable of supporting the weight of the turbocharger and other forces introduced by the turbocharger to the engine.
One attempt at designing an exhaust manifold flange to reduce the incidence of cracking of the flange is illustrated in U.S. Pat. No. 5,406,795 (“the '795 patent”) issued to Raub et al. on Apr. 18, 1995. The flange disclosed in the '795 patent has two exhaust ports. The two exhaust ports are generally trapezoidal in shape and are separated by a thin straight center wall. Experimentation has shown the flange is not capable of handling the increased temperature of the exhaust gases produced by today's internal combustion engines. The thermal energy destroys the center wall. Therefore, an exhaust system is needed that combines the exhaust manifold, the exhaust manifold flange and the turbocharger permitting a rigid connection between the flange and the turbocharger and reducing the thermal energy absorbed by the manifold and the flange. Thus, the operating life of both the flange and the turbocharger is increased, the efficiency of the engine is improved, and the power output of the engine is increased.